Methods have been developed to allow the evaluation of neurotoxicity by testing for changes in neurotransmitter-related biochemistry in treated animals. A series of transmitter receptor sites can now be assayed; as can monoamines and their catabolic products by high performance liquid chromatography. Other techniques include radioimmunoassay of circulating endocrine hormones and of neuropeptides. By such approaches, the following factors have been identified as potential contaminants or modulators of the neurochemical response: (a) Prior handling experience, (b) sex of animal, (c) the vehicle in which the toxic agent is administered, (d) the age of the animal, and (e) the time of day at which the animal is treated or killed. In addition, asymmetric distribution of several biochemical parameters has been found between left and right brain regions. Using acrylamide as a model compound, evidence suggests that toxicants can influence cerebral chemistry (a) directly, (b) by way of the endocrine system, and (c) after conversion by hepatic enzymes to a toxic catabolite. A relation between changes in the dopamine system and altered behavior after a pharmacological agent was suggested. Studies on manganese-treated rats show receptor binding assays may be used to detect selective damage to a specific neuronal circuit.